In this randomized, double-blind, placebo-controlled, multiple-dose, 2-period, crossover study of 12 obese subjects, we examined the effect of RM-493 on resting energy expenditure in a room calorimeter. Twelve healthy adults (6 M, 6 F) with BMI 35.7 2.9 kg/m2 (mean, SD) received RM-493 (1 mg/24 h) or placebo by continuous subcutaneous infusion over 72 hours, followed immediately by crossover to the alternate treatment. All subjects received a weight-maintenance diet (50% carbohydrate, 30% fat, 20% protein) and performed 30 minutes of standardized exercise daily. Continuous EE was measured on the third treatment day in a room calorimeter and REE in the fasting state was defined as the mean of two 30-minute resting periods. RM-493 increased REE vs. placebo by 6.4% (95% CI: 0.68 to 13.02 %), on average by 111 kcal/24h (95% CI: 15 to 207 kcal, p=0.03). Total daily EE trended higher while the thermic effect of a test meal and exercise EE did not differ significantly. The 23-h non-exercise respiratory quotient was lower during RM-493 treatment (0.833 0.021 vs. 0.848 0.022, p=0.02). None of our subjects had exon nucleotide variants of MC4R associate with obesity. Recently, MC4R receptor activation in intestinal epithelial L-cells has been shown to acutely increase GLP-1 and PYY secretion in vivo in mice, and in colon explants from both mice and human. Similar to the rodent data, fasting total GLP-1 and PYY levels were slightly, but significantly increased during RM-493 administration in our study. Thus, it is possible that MC4R-induced GLP-1 production may contribute to eventual beneficial effects on insulin and glucose, while PYY (assumed to be PYY3-36) can positively impact energy metabolism during obesity treatment. It is possible that increases in both peptides may contribute to the weight loss effect of RM-493 seen in animals. No adverse effect on heart rate or blood pressure was observed.